1. Field of the Invention
The present invention relates to an emulsion of a crosslinking type comprising a polymer and an oligoester-(meth)acrylate as a crosslinking agent. In the subject disclosure, acrylate and/or methacrylate are referred to as "(meth)acrylate", acryloyl group and/or methacryloyl group are referred to as "(meth)acryloyl group", and acrylic acid and methacrylic acid are referred to as "(meth)acrylic acid".
2. Description of the Prior Art
Heretofore polymers, such as vinyl polymers, have been dissolved in or mixed with an oligoester-(meth)acrylate and irradiated with an ionizable radiation or heated with a peroxide for cure by crosslinking. However, these curing methods give a very high viscosity to the composition or yield a solid composition, and therefore, properties such as handling, coating procedure and the like are poor. From the point of view of solubility, the polymer employed has been restricted to that of a relatively low molecular weight and thereby the resulting cured matter has only a limited range of physical properties, and requires a lot of an organic solvent or the monomer for dilution.
On the other hand, a polymer emulsion of an oil-in-water type obtained by an emulsion polymerization is a water dispersion of a polymer of a very high molecular weight. A coating formed from the high polymer has generally a high flexibility and a good adherability, and since the emulsion contains water as a dispersion medium air pollution problems, such as may be due to evaporation of organic solvents, are avoided. However, it is difficult to obtain a high hardness because a high hardness tends to deteriorate a film-shapeability and further, the high polymer is not a crosslinking type and therefore the solvent resistance is poor. In view of the foregoing, many attempts have been made to crosslink the emulsion.
In case of conventional polymer emulsions of a crosslinking type, when an acrylic acid ester is subjected to an emulsion polymerization, a hydroxymethylated amido group or a carbonyl group is introduced into the polymer side chain by copolymerizing with a polymerizable monomer having a hydroxymethylated amido group or a carbonyl group, and the polymerization mixture is heated, after the dispersion medium is evaporated, to cause a self crosslinking or heat-crosslinking by using melamine as a crosslinking agent.
Therefore, there are the following disadvantages.
(1) Formaldehyde is generated upon the heat crosslinking giving rise to a possible environmental pollution problem.
(2) The heating temperature is high and the heating time is long.
(3) Unreacted hydroxymethylated amido group or carbonyl group remaining after the heat reaction lowers a water resistance of the crosslinked polymer.
(4) Upon emulsion polymerization, a particular polymerizable monomer having functional groups for crosslinking should be used.
(5) Increasing a hardness of the coated film is difficult.
The subject invention eliminates the above disadvantages of a polymer emulsion of an oil-in-water type, and in accordance therewith it has been found that the oligoester-(meth)acrylates which are inherently oil-soluble deposit on emulsion particles, and the emulsion particles and the oligoester-(meth)acrylates are mutually dissolved without destroying the polymer emulsion of an oil-in-water emulsion by a simple procedure, and the resulting emulsion of a crosslinking type shows excellent synergistic effects as shown below.